The present invention relates to a solenoid including permanent magnets arranged for bistable actuation in combination with a knitting machine employing the same.
A known bistable solenoid is provided with a yoke having two permanent magnets arranged on opposite sides of a magnetizing coil and a movable ferrous core which has an overall length shorter than the distance between the outer ends of their respective permanent magnets and is movably fitted into the yoke, as disclosed in Japanese Patent Laid-open publication 56-26127 (1981 ) or Utility-model Laid-open Publication 54-35314 (1979).
A disadvantage is that an accurate positioning of the movable ferrous core at a desired position is troublesome.
For the purpose of eliminating a disadvantage, a modification has been proposed as shown in Japanese Utility-model Laid-open Publication 63-188910 (1988).
As best shown in FIG. 6, the modification comprises a solenoid enclosure P11, a couple of magnetizing coils P13 and P14 sandwiching therebetween a permanent magnet P12 which is magnetized in radial polarity orientation, two end plates P15 and P16 arranged on the outer sides of the two magnetizing coils P13 and P14 respectively, and a cylindrical sleeve P17 extending outward across the two end plates P15 and P16. Accordingly, there are developed a pair of left and right magnetic coils P13 and P14 between the center permanent magnet P12 and the two end plates P15 and P16, respectively. The cylindrical sleeve P17 accommodates a movable iron core P18 which extends lengthwise of the sleeve P17 and has two interacting regions P19 and P20 arranged equal in width to their respective end plates P15 and P16. Also, a couple of small-diameter regions P21 and P22 of the movable iron core P18 are formed inside their respective interacting regions P19 and P20.
Since the small-diameter regions P21 and P22 of the movable iron core P18 are smaller in permeability than the other regions, the movable iron core P18 becomes stable when either of its interacting regions P19 and P20 meets the corresponding end plate P15 or P16. Also, the thickness of the end plate P15 or P16 is identical to the width of the interacting region P19 or P20 so that the positioning of the movable iron core P18 can be ensured.
A cam drive mechanism of a knitting machine using such a known solenoid is illustrated in FIG. 7. The solenoid P2 is fixedly mounted by a retaining member P3 to a base plate P1. A movable plunger P6 of the solenoid P2 is provided for pressing upward one end of a rocking lever P5 pivotably supported by a support P4. The other end of the rocking lever P5 is arranged for actuating a lift-down cam P7 or the like. When the solenoid P2 is deenergized, its moving plunger P6 remains retracted by means of a spring.
Also, disclosed in Japanese Patent Laid-open Publication 57-29649 (1982) is a cam supporting carriage of a knitting machine which carries a movable cam actuated by an electromagnetic positioning means for outward and inward movement to control the action of knitting needles. The electromagnet positioning means comprises a permanent magnet exhibiting a small magnetic field and arranged in combination with coils for magnetization and demagnetization and a moving unit of ferromagnetic metal material linked to the cam to be positioned. Also, a magnetization control circuit is provided for allowing the coils to perform a magnetizing and demagnetizing action on the permanent magnet using current pulses. As a result, both the moving unit and the movable cam linked with the moving unit can be actuated by the action of magnetic attraction and repulsion for cam engagement and disengagement.
However, the foregoing solenoid described in Japanese Utility-model Laid-open Publication 63-188190 still has a drawback that the movable iron core slides directly on the inner surface of the cylindrical sleeve and thus, both will unavoidably be worn away. Particularly, the movable iron core is made of soft iron for enhancement of magnetic characteristics having a low resistance to wear.
The magnetic circuit extends up to the end plates where there are slight clearances between the cylindrical sleeve and the interacting regions of the movable iron core. Hence, the magnetic flux tends to leak out and attract unwanted materials, e.g., existing iron dust. Such iron dust may enter inside the sleeve and accelerate the wear of both the movable iron core and the cylindrical sleeve.
The clearance between the cylindrical sleeve and the interacting regions of the movable iron core has to be determined to a minimum distance for minimizing the entrance of iron dust and the end plates are not allowed to act as bearing bushes.
Also, if the magnetic intensity of the permanent magnet is increased for increasing a force of retention, a greater energy of flux develops across the magnetic circuit. Simultaneously, the magnetic circuit causing unstable conditions is also increased in the magnetic energy. As a result, the permanent magnet and/or the magnetizing coils have to be increased in size for producing appropriate rates of retention force and thrust force while the moving distance of the movable iron core has been set to a desired length.
The foregoing known knitting machine employs a multiplicity of such solenoids which produce a thrust of 1 kgf for actuating each lift-down cam which can be driven by a thrust as small as 300 gf.
The 1-kgf solenoid produces not only a greater thrust but also an unwanted physical impact causing noise and vibration during operation of the knitting machine and the operational durability will be declined. The size of the solenoid has to be increased proportional to the magnitude of a thrust and will never contribute to the compactness of the knitting machine.
Furthermore, the foregoing solenoid used for actuating the cam in a knitting machine, as shown in FIG. 7, has to be accompanied with the rocking lever P5 for cam actuation, the retaining member P3, the support P4. etc. Accordingly, the cam drive arrangement is complicated and hardly decreased in size. Also, the mass of inertia of moving parts becomes great, thus discouraging high-speed operation and requiring large magnetizing power.
In addition, the adjustment on the clearance at the stress and action points of the lever P7 has to be carefully carried out, which is troublesome.
The solenoid of the cam supporting carriage disclosed in Japanese Patent Laid-open Publication 57-29649 contains a single permanent magnet and is thus provided with a spring which produces a counter-force for bistable movement. For drawing the iron core, a greater force of magnetic attraction is needed than the yielding force of the spring. This results in declination in the efficiency of energy conversion. Also, during returning of the cam to its actuating position, the iron core is abruptly pressed outward by the yielding force of the spring, thus producing a physical shock which may accelerate the wear of the iron core and its relevant components.
It is then an objection of the present invention to provide an improved bistable solenoid and a knitting machine using the same.